PC Low and PC Ultra share identical assets.
They run on the same or different hardware at varying framerates... however the differences between PC low and PC ultra go beyond merely resolution and frame-rate. Generally. Across the PC industry. Also specifically for the Witcher as can be seen from the XML configuration files in our installs.
Merely having "the same assets" does not come close to meaning "is the same". Once I have the game running I will try both PC Low and PC ultra, before settling on a set of high/ultra that will work tolerably well with my machine. I will be happy, because I would be happy to play what I have seen of the PS4 and XB1 footage. I also think it will be noticeably nicer than those.
I will find the 2000 doors of Novigrad and I will entertain myself for a few minutes going in and out of a variety, a small sampling, of these while not experiencing any loading screens.... and I will giggle, and then go back to finding things to chop up, people to talk to and other fun things.